Multistage hydraulic pressure boosters for use in vehicular braking systems are relatively well known in the prior art. Such pressure boosters, or intensifiers, employ a system of concentric pistons to obtain a gradual boost in wheel cylinder pressure, providing a brake pedal feel comparable to that provided by power assisted vehicular braking systems. Such prior art pressure intensifiers are illustrated in the U.S. Pat. Nos. 3,101,282 and 3,010,238 to Jansson, and the U.S. Pat. No. 3,425,222 to Cooney, issued Feb. 4, 1969. These references disclose a self-contained apparatus installed along the brake fluid line extending between the master cylinder and the wheel cylinders of a vehicle. A cylindrical chamber surrounds two concentric pistons slideably disposed in the cylinder. A compression spring biases the two pistons toward a fluid inlet end of the cylinder. A spring-loaded ball valve is disposed inside the inner piston.
The brake pressure intensifier is actuated by fluid pressure delivered from the master cylinder. When an operator of the vehicle applies force to the brake pedal, fluid pressure from the master cylinder is transmitted to the inlet end of the pressure intensifier apparatus, moving the brake fluid straight through both concentric pistons and the check valve to the wheel cylinders without providing any pressure intensification. When a predetermined level of pressure is exerted from the master cylinder, the two concentric pistons move together against the biasing pressure of the spring and allow the ball valve to close. This movement gradually increases the fluid pressure due to the fact that the inlet ends of each of the two concentric pistons are larger in diameter than at their respective outlet ends. As pressure increases in the master cylinder, the two pistons continue to move until the outer piston is stopped by an annular ledge extending inwardly from the cylinder. The inner piston, however, is not so restrained and continues to move in the cylinder.
As shown in the above-referenced prior art patents, a spherical valve member is provided in the ball valve. It has been found that at high pressures, fluid leakage is experienced between the spherical ball and its associated seat surface in the flow passage. This fluid leakage results in decreased operating efficiency and a reduction in the pressure intensifying characteristics.